text:
What does ATC do when a VFR flight in the mode C veil has a malfunctioning altitude encoder or transponder?
Recently I replaced the altitude encoder in my airplane because ATC could not receive my altitude during VFR flight following. Up to that point I had been flying VFR under the class B shelf for some time, and I assume my transponder wasn't transmitting the altitude to ATC. It may have been non-functional for several months.
Was I causing panic in the control tower because my plane wasn't reporting the altitude, and I wasn't on any ATC frequencies for them to contact me? Can ATC determine my altitude from their ground based radar? Is ATC able to contact VFR airplanes when their transponders/encoders are off, or tuned to the wrong squawk code, or malfunctioning? The transponder is tested every two years, so that is a lot of time for something to go wrong.

Where are you flying?

I'm flying in the MSP area.

ATC may ask for your altitude and want to be notified of altitude changes

A lot of this has to do with whether or not you're talking to ATC.
If you're talking to ATC and your Mode C disappears or goes insane they'll ask you to report your altitude (and if your Mode C altitude reporting is wildly off they'll probably tell you something like "Stop altitude squawk - altitude differs by 500 feet". This is your cue to take the plane to an avionics shop and have the system checked/serviced, and not to attempt to enter B or C airspace, or fly above 10,000MSL, until its fixed.
In most cases ATC will continue to work you in the system until you are clear of the Mode C veil (possibly expediting your exit if having no altitude readout on your target is messing up their day) because it's better to be talking to you than not.

If you're not talking to ATC things get a bit more complex:
If your Mode C altitude readout is missing ATC will generally report you as "traffic, altitude unknown" to other aircraft and apply lateral separation exclusively (nobody is allowed within the minimum lateral separation distance of your target unless they have you in sight and can maintain visual separation). If/When other pilots spot you ATC will usually ask them for your type and altitude so they have some idea of who you are and what you're doing.
If your encoder or transponder is malfunctioning and reporting an incorrect altitude that's substantially more dangerous: ATC has no way of verifying your altitude other than asking you (or another pilot), so they'll be maintaining separation based on an incorrect altitude. This is why even VFR aircraft equipped with Mode C need an encoder check.
In this case ATC will find out about a malfunction when you're called as traffic to someone and they respond with "Is that my traffic above/below me?"

That's why you'll sometimes hear a traffic report as "Traffic, two o'clock, one mile, southbound, altitude indicates 3,500". ATC is telling you that they have a mode C altitude, but no idea how accurate it is.


text:
What property of a wing allows it to flex?
Is it the material used that allows the wing to flex or is wing flex caused by the structure of the wing?

pure physics. no material will ever be able to prevent a wing to flex.

Flexibility. The clue is in the name!

The amount a wing can flex is actually a product of the material. The wing requires a specified ultimate strength; with metal, that translates into a given amount of flex. This can be varied within limits, but it is really the material, its stiffness to yield point ratio, and its fatigue properties, that control how much a wing will flex.
Carbon-fiber-reinforced polymer is an extremely strong and light material and has lesser stiffness, and has essentially no fatigue problems. This is beneficial to provider a smoother ride in turbulence. The wing acts like a big spring.
As Federico mentioned, any material can and will flex (to a varying degree). So flexing is because of the material of the wing more than the wing itself.

The flexing of a structure like an aircraft's wing is generally due to the materials used, but is also related to the nature of the structure itself.
In very simplified form, a wing is a beam that is supported at one end (the fuselage). Any forces such as from aerodynamics that act on the beam will cause it to bend.
All materials have a characteristic way in which they behave when load is applied. This can be expressed as stress (from applied forces) vs. strain (deformation of the material). Once the material reaches the yield strength point, it will be permanently deformed. For stress below this point, though, the material will return to its original shape. The wing can then flex as long as it remains below the yeild strength in all required loads.

See here for more about this graph
Of course there is much more detail to the subject. The whole structure is designed together so that it will be as light as possible, but the amount of flexing is limited to prevent related issues. The wing may need to be made more stiff to prevent flutter. Farhan mentions fatigue, which can also limit the amount the wing may flex. The panels on top and bottom of the wing must also be prevented from breaking or permanently buckling. There are also fuel tank seals that should not be broken. Adding material in the right places can help to spread out the loads, which reduces the stress on the material and therefore reduces the amount of flexing.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

authors: Руслан Волошин

sort: votes


text:
What is the purpose of flutter testing?
All I am aware of is that flutter testing involves the aircraft shaking violently. How is a flutter test performed and what is its purpose?

Flutter is a phenomenon that can occur when a structure is subjected to aerodynamic forces. It occurs not only in aircraft but also for example in buildings, power lines, road signs and bridges. Flutter is an oscillation caused by interaction of aerodynamic forces, structural elasticity and inertial effects.
Perhaps the best known example of flutter is the bridge over the Tacoma Narrows ('Gallopin' Gertie')  that collapsed due to flutter in gale force winds.

Flutter in aircraft causes the wings and / or stabilizers to oscillate.
When the airspeed increases the energy added in each oscillation to the structure by the aerodynamic forces increases. At some speed the damping of the structure may be insufficient to absorb the energy increase from the aerodynamic loads and the amplitude of the harmonic oscillations will grow until the structure breaks.
A flutter test is performed to determine what speed flutter occurs yet is still damped. Theoretically an aircraft should be able to stay structurally intact up to the design dive speed ($V_d$) which is at least 1.4 times the design cruise speed. In flight tests, the $V_d$ is not always demonstrated as this is very risky since there would not be any margin from breaking up. The maximum speed is slowly increased in subsequent flight tests with careful data analyses in between up to a point where the test team decides that it is no longer safe to continue. This establishes the maximum demonstrated dive speed ($V_{df}$); the published maximum operating speed ($V_{mo}$) and never-exceed speed ($V_{ne}$) are well below $V_{df}$ to give margin from disaster.
This video shows flutter in a Hawker 800. The incident occurred not during testing but was caused by inadequate fitting of the ailerons(FAA PDF).

Aeroelastic flutter is a dynamic instability of an elastic structure where there is feeding between the deformation of the structure and the force provided by the aerodynamics.
Is a very dangerous condition where the vibration on the structure keeps growing and so the stress over it, endangering safe flight. It is a condition that needs to be completely avoided in a commercial flight.
Airplane manufacturers need to provide information to airlines of the operation envelope of the airplane, and part of that information is to include region of flutter to be avoided (or maximum time on it).
For providing such information manufacturers predict using internal methods (usually simulation) the flutter speed for each condition, but certification authorities request that a physical test is perform to detect the flutter velocity which will be the final speed provided in the airplane manuals.
How is made? Taking an specific airplane weight the flight test pilots approach steadily step by step to the flutter speed until certain level of vibration is obtained, determining the final airplane flutter speed.

Flutter needs to be avoided in any kind of flight @TrebiaProject!

@GdD but needs to be detected in Flight testing....

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text:
What is Brake To Vacate (BTV)?
Emirates has this feature on many of its A380s. From what I've read, the feature decreases the amount of runway an aircraft uses after landing. It also helps the aircraft achieve a certain exit way. Is this feature unique with only Aibus aircraft? How is it set up on an aircraft and how does BTV work?

According to the Airbus website, Brake to Vacate is a feature installed on newer A-380s and also on the new A-350XWB that allows the pilot to select which runway exit s/he needs to use, and the system will brake hard enough, but just hard enough, to slow the plane to taxi speed by that turnoff.  This minimizes time on the runway.  The following quote is from the Airbus website.

Brake to Vacate allows pilots to select the appropriate runway exit during the approach to landing, and regulates the aircraft’s deceleration after touchdown – enabling it to reach any chosen exit at the correct speed under optimum conditions, no matter the weather and visibility.

To my knowledge, Airbus is the only aircraft manufacturer to incorporate this feature - to date.
This feature does not reduce the amount of runway the aircraft uses.  Maximum Braking is still maximum braking.  It does however, optimize the braking so that the aircraft gets to the taxiway intersection sooner (doesn't brake too hard too soon, keeping speed up) and also prevents overshooting the intersection requiring a relatively slow taxi to the next turn off.

The predecessor to BTV is simply AutoBrakes, where a level of braking can be selected in the cockpit, each level corresponding to a deceleration level.  (On some Boeings, A/B 1 is very mild, A/B 2 is a fairly normal stop, A/B 3 is moderately firm, and A/B 4 is extremely firm braking, enough to get AntiSkid cycling & producing an uncomfortably rapid stop on a dry runway.) While pilots can override the autobrakes, you do that based on visual cues of how rapidly you need to slow to make "that' turnoff. BTV automates that process, aiming to avoid slowing too soon or braking too firmly at the end.

Does this mean on the A380 there is no autobrake function at all, and its completely replaced by BTV? Do you expect it to be introduced on smaller aircraft in the future? I am trying to understand is BTV something necessitated by the size of the A380 or its upgraded stopping requirements, or is this a general improvement in overall autobraking technology that may be seen in other aircraft in the future?

@BurhanKhalid BTV is an enhancement to the existing autobrake system, so the regular Airbus autobrake settings would also be available. It's entirely a convenience for the pilots, so in theory any plane with an autobrake system could have something similar.

In addition to previously mentioned, BTV also reduces brake wear, because it optimizes the usage of wheelbrakes to as late as possible. This way aerodynamic braking and reverse thrust slows the aircraft first and wheelbrakes are used as little (and late) as possible.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is the significance of a squawk code?
I've been looking for a detailed explanation for how the transponder works and the importance of a squawk code.

What have you found out so far? Are you looking for instructions for a specific make and model of transponder? Do you want to know what are the common squawk codes and their meanings? Do you want to know how ATC reassign squawk codes? Please clarify - I am voting to close in meantime.

Actually, I was hoping for an answer that addressed all of that, or at least an answer that talks about the way ATCs reassign squawk codes and how they are made visible to ATCs in the first place.

Start at Wikipedia - In my opinion, that's a lot of material to reproduce here.

The squawk code is assigned by Air Traffic Control to the aircraft and is used for radar identification purposes. It is a 4 digit octal number (each digit has a value in the range [0-7]).
The flight crew enters the assigned squawk code into the transponder control panel and the radar extracts the code using a Mode-A interrogation (or UF5 / UF21 interrogation in case of a Mode S radar/ transponder).
The extracted squawk code can then be used to correlate the radar reply with an aircraft's flight plan. A number of squawk codes are reserved for generic purposes and emergencies. These codes are used in emergency, or when no flight plan correlation is needed. Some generic codes are:

1000 - Conspicuity code signalling the Flight Data Processing system to use Mode-S Aircraft ID for flight plan correlation. (Primarily in Europe)
1200 - Visual Flight Rules standard squawk code (USA & Canada)
2000 - Used when entering a Secondary Surveillance Area and no code has yet been assigned.
7000 - Visual Flight Rules standard squawk code (ICAO, USA & Canada use
1200 instead)
7500 - Unlawful interference / hijack (ICAO, worldwide)
7600 - Radio communication lost (ICAO, worldwide)
7700 - General emergency (ICAO, worldwide)

Also, ATC squawk codes while seemingly random may have a meaning within that particular facility, but there is not exactly a standard or meaning useful to anyone but ATC.  For example, "local" flights may be assigned codes in the 0xxx range, but in the US, the squawk code is automatically determined by the ATC system.  Occasionally conflicts can occur, and you'll hear ATC direct to "reset transponder, squawk xxxx".

It allows Secondary radar to identify each aircraft. The response ping from the aircraft contains the 12 bit squawk code.
Each plane is assigned a different squawk code as they enter an airspace and the software compares that to the flight plans filed and then can display the relevant information on the display.
If you fly VFR without a flight plan you will squawk the general VFR code until asked to change it when entering controlled airspace. When leaving tghe controlled airspace you will be asked to "squawk VFR" again.
They can also be used to declare emergency by squawking 7700, 7600 or 7500 (for general emergency, radio loss and hijack resp.)

75, taken alive.  76, in a fix.  77, going to heaven


text:
How do pilots board the B-1B Lancer?
I've noticed that there isn't any type of hatch/plug door near the canopy of the B-1B Lancer. How do pilots enter and exit the aircraft?

The crew of Rockwell B-1 Lancer (4 in total) enter or exit the cockpit from the underside, using a ladder. It can be seen in this video.
This picture shows the ladder (the one on the right):

Image Source
On the top of the ladder, there is a door, which looks like this:

Image Source

That's not a ladder -- it's a bookshelf!

This picture shows a ladder behind the front landing gear.

Image Source

Why is it situated behind the landing gear?

@MadhavSudarshan not enough room in front (the gear retracts forwards) and symmetry

@MadhavSudarshan If there are crew riding on the jump seats, they do not have ejection seats and the bail out is manual through this hatch.  If the hatch was in front of the nose wheel, this would be a problem if they had to bail out when the gear is down.

I certainly wouldn't want to be in one of those jump seats if the pilots thought ejecting was a good idea. They're risky enough in multi-crew aircraft even when everyone has one, especially in side-by-side configuration (one reason the F-111 used cabin ejection instead of rocket seats; this design was also considered for the B-1).

The B-1A did have an escape capsule.  It was changed to ejection seats for the B-1B.  http://www.check-six.com/Crash_Sites/B-1_crash_site.htm


text:
How do pilots determine the takeoff distance of an aircraft?
While making a flight plan, depending upon the weight of the aircraft, I'm assuming pilots determine the takeoff distance. How do they do so (is there a formula to calculate the takeoff distance)?

Related: What would be the ground roll and total distance to clear a 50ft obstacle given these conditions?

See here for a practical example

Are pilots to determine this? What about passengers and airline companies?

Most airplanes have a POH (Pilot Operating Handbook) with a table on aircraft distances.  Here I have included one for the C172.

Takeoff data is calculated using performance charts and/or software.  Data input into the algorithm include:

runway condition
runway slope,
wind (headwind or tailwind component),
takeoff weight,
aircraft configuration (flaps, slats, etc)
density altitude

The air carrier I flew for had a performance chart for every runway we operated from (all scheduled and alternate destinations) that we could reference for the max allowable takeoff weight for that runway, flap configuration, wind condition and temperature.  We didn't actually calculate the takeoff roll distance, but rather that we were below the max weight that could accelerate to V1 and stop within the distance provided by the runway in the current wind, temperature and aircraft configuration.

The previous answers are indeed correct; I will take it one step farther up the stack, though, and make a point that may have been unclear and addresses an assumption.
The pilot doesn't calculate take-off distances; they are looked up.
As mentioned, external data sources are used. The manufacturer of the aircraft, based upon their own math and the results of real-world-testing, is the authoritative source. Once it's in the operating handbook, you can be confident of those numbers (and, in fact, shouldn't second-guess them without a bloody good reason). As was also mentioned, some operators do their own testing; these results then go into the operating handbook, and the pilot is then expected to be able to count on those numbers as long as they follow the same procedure.
One of the most important aspects of flight is consistency; given the same external conditions, all aircraft of a particular model and particular takeoff weight should perform very similarly. This is more than just minimums. In the case of a broken instrument, whatever is being measured by the broken instrument can usually be inferred from other instruments. An example would be airspeed; if your airspeed indicator is broken, but you know your engine RPM and know that you're not climbing or descending, then you should always be about the same airspeed given that engine speed. A Cessna 172S would always be very near 120kts at 2400rpm at 2200 pounds weight. This consistency is what allows those numbers to be recovered and depended upon. (pulling numbers from head; the point is that you should be able to go into ANY Cessna 172S and expect the same performance as long as the plane has been properly maintained).
This is also true of many other critical aspects of an aircraft's performance. Climb rates, fuel consumption, rate of change-of-direction in a turn... these numbers all have to be accurate, as they can mean the difference between making the runway and becoming a news story.

Besides using tables, aircraft manufacturers also provide a takeoff chart, like this:

The chart goes from left to right:

Enter the air temperature, then go up until you met your pressure
altitude. Then proceed horizontally to the right.
Follow the contour of the lines, until you meet your weight on the
x-axis. Proceed horizontally to the right.
Follow the contour of the lines, until you meet your headwind (or
tailwind) component in the x-axis. There, leave the contour and go
horizontally to the right.
The y-axis gives your takeoff roll distance. If there is an obstacle
50 feet at the end of the runway, follow the lines to get your
"takeoff roll + clear obstacle" ground distance.

Some manufacturers provide tables. Some provide charts. Some do both.

The way we teach at our school is that, for example, if you are doing a normal takeoff and your POH only has short field effort, take the Vr and the Vlof, and find the difference. For a C172sp that is roughly 9kts difference. So now take your normal takeoff speed, and add that difference for your Vlof normal. You'll find that it's almost a 10% increase in speed.  As a rule of thumb, a 10% increase in speed will equal about a 21% increase in distance.


text:
How is it possible for manufacturers to constantly get data from their planes all over the world?
I've heard that manufacturers (like Boeing and Airbus) constantly get data from their planes. So in the case of an emergency, ATC's or pilots can contact the manufacturers for assistance. How do manufacturers constantly get this data and how is it stored (cause considering the number of planes flying today, it must be a lot)?

Every purchase you and everyone else make at Walmart, Tesco, etc is stored centrally. There are a lot more purchases than planes.

How do makers get data from aircraft in flight
The data is usually transmitted using something like ACARS which is a system used in aircraft to transmit data in flight using underlying radio and satellite communications systems.
The communications infrastructure is typically provided by large multinational companies who specialise in this job. See ARINC, SITA, Inmarsat and so on.
Engine-makers and Aircraft makers often offer pro-active maintenance and support services to airlines, operators and lessors which involve their aircraft transmitting such data to the makers.
For example, Airbus has AIRTHM

Airbus Real Time Health Monitoring (AiRTHM) is an advanced service through which A380 and A350 XWB operators can receive guidance on optimised maintenance and real-time troubleshooting actions. As part of this effort, a dedicated 24/7 team takes advantage of the uplink technology to further investigate and anticipate warnings/fault consequences.
The uplink technology allows real-time remote access to aircraft data parameters via the ACARS digital datalink system, enabling Airbus AiRTHM engineers to deliver maintenance and technical advice both in flight and on the ground.

Boeing have AHM

AHM sutomatically monitors, collects and transmits service levels using ACARS through the installed Aircraft Condition Monitoring System (ACMS)

Note that ACARS is a relatively low-volume data transmission system. Makers have explored other underlying communication systems as a way to transmit more data at lower cost.
How is this data stored
It is true that there is a lot of data but Boeing and Airbus are big businesses and are able to partner with major IT businesses.
For example IBM work with Airbus

“Today’s aircraft can generate up to a half terabyte of data per flight, an unprecedented volume and variety of data seen in few other industries,” said Timothy Wholey, IBM global leader for the aerospace and defense industry in its global business services unit.

Note - not all data has to be transmitted in flight. A lot of data is collected and stored in the plane and only transferred when the aircraft is on the ground.
Makers like Airbus and Boeing inevitably are already very heavy users of IT for aircraft design purposes. For example, Airbus use HP Supercomputing hardware

They are installed with some sensors which keep sending important data to the manufacturer. And, talking about their size, they are merely in just KBs, which is very small compared to storage the amount available even for all flights combined. Also, all information collection is automatic i.e. handled by computers so there is no problem in collecting information either.
Read: http://www.forbes.com/sites/johngoglia/2014/03/13/aircraft-engine-monitoring-how-it-works-and-how-it-could-help-malaysia-air-370-crash-investigtors/

Boeing has a system installed in many airplanes which monitors engine health. It is called Airplane Health Management. A good presentation can be seen here (more info here and here).
Airbus uses Airbus Real Time Health Monitoring (AiRTHM) which essentially does the same (more info here).
Please note that these systems are not installed on every single airplane but are getting popular more and more.


text:
Why did FAA regulations initially prevent passengers from making phone calls during a flight?
For many years, the FAA prevented people from making phone calls during a flight. Was this because a phone call interfered with aircraft communications?

As far as I know, the FAA doesn't prohibit (and did not in the past) prevent people from making phone calls during a flight. The FCC did prohibit the use of mobile phones on aircraft to protect the cell phone network from fast moving transmitters that are in the range of ground antennas at the same time.

I personally remain unconvinced that they are suddenly now safe.  A good friend of mine is the avionics trainer for my local airline.  He knows when phones get signal in the descent and negotiate with the cell towers as he can hear the distinctive chirping in his headset.  He says it's just an annoyance.  I disagree with him.  Jetfast 123, immediate chirp crackle buzz pop turn, avoiding action.  Should he go left or right?

@Simon, how do airlines like Emirates and Delta allow in-flight phone calls?

@MadhavSudarshan Do you mean inflight calls or using mobile phones?

@Simon, thank you for pointing that out, I meant mobile phones.

While there is a chance it can interfere with aircraft equipment there are lots of other reasons as well.

It's disturbing, you would not want a can full of people yapping on their phones for 5 hours while others tried to sleep. Lets be honest does asking someone to "Keep It Down" ever really work.
It's very taxing on the cell network itself. Without going into lots of detail about how cell phones work, the short and sweet of it is that when you are moving (say in a car) your phone is constantly hopping form tower to tower. At the speeds a plane moves at you would hop tower at a rate that would be far more taxing on the system than when you are in a car and could lead to dropped calls and other connection issues.
You need to listen up! While the laws on in flight entertainment have become more relaxed over time, one of the reasons you are not to use things during takeoff and landing is these are critical times in the flight (for something to go wrong) and if it does, you need to be attentive and able to listen to instructions. This, on some level, holds true for the whole flight as well I would think.
The phone itself occupies its own frequency band outside of the band that aircraft radios use however there is always a possibility when you have 2 EM waves that interference will occur.

The use of personal electronic devices (including cell phones) during flight is still generally prohibited by the FAA, unless the operator or PIC allows it. FAR 91.21 the current wording on the use of electronic devices

§91.21   Portable electronic devices.
  (a) Except as provided in paragraph (b) of this section, no person may operate, nor may any operator or pilot in command of an aircraft allow the operation of, any portable electronic device on any of the following U.S.-registered civil aircraft:
(1) Aircraft operated by a holder of an air carrier operating
  certificate or an operating certificate; or
(2) Any other aircraft while it is operated under IFR.
(b) Paragraph (a) of this section does not apply to—
(1) Portable voice recorders;
(2) Hearing aids;
(3) Heart pacemakers;
(4) Electric shavers; or
(5) Any other portable electronic device that the operator of the
  aircraft has determined will not cause interference with the
  navigation or communication system of the aircraft on which it is to
  be used.
(c) In the case of an aircraft operated by a holder of an air carrier
  operating certificate or an operating certificate, the determination
  required by paragraph (b)(5) of this section shall be made by that
  operator of the aircraft on which the particular device is to be used.
  In the case of other aircraft, the determination may be made by the
  pilot in command or other operator of the aircraft.

Advisory Circular 91-21-1C provides further details on how one may comply with this section and also advises that per FCC regulations part 22.925, the use of cell phones while airborne is prohibited due to the potential for interference with ground-based communications networks (cell phone signals travel much farther from an airborne source vs. one the ground one) UNLESS the aircraft has special equipment installed in order to help reduce signal interference.

The prohibition on making phone calls is the domain of the Federal Communications Commission (FCC), not the FAA (the FAA has blanket bans on portable electronic devices but not the actual use to make phone calls).
The FCC regulation question is 47 CFR 22.925:

§22.925   Prohibition on airborne operation of cellular telephones.
Cellular telephones installed in or carried aboard airplanes, balloons or any other type of aircraft must not be operated while such aircraft are airborne (not touching the ground). When any aircraft leaves the ground, all cellular telephones on board that aircraft must be turned off. The following notice must be posted on or near each cellular telephone installed in any aircraft:
“The use of cellular telephones while this aircraft is airborne is prohibited by FCC rules, and the violation of this rule could result in suspension of service and/or a fine. The use of cellular telephones while this aircraft is on the ground is subject to FAA regulations.”

As far as I'm aware the original reasoning for this prohibition of airborne cellular phone usage dates back to the original analog cellular networks in the US.  The old analog system had issues when your phone could communicate with many towers at once and this was the case if you put the phone at some altitude above the ground giving clear line of sight to many towers.  This caused problems not only for the phone but also the network.
Current digital cell networks do not have these kind of problems but for the most part are unusable at altitude due to design decisions of the network (primary radiation lobes are horizontal and sometimes down tilted relative to the horizon, not pointing up into the sky).  There are also issues with speed that make communicating with ground based towers possible (if you can talk to them at all).
An excerpt from the airborne cell NPRM:

6.
  Part 22 of the Commission’s rules prohibits the airborne use of 800 MHz cellular
  telephones, including the use of such phones on commercial and private aircraft. This prohibition was
  adopted in 1991 to guard against the threat of harmful interference from airborne use of cellular phones to
  terrestrial cellular networks. The Commission’s prohibition was not to ensure interference-free operation of avionics equipment. When the prohibition was adopted, the Commission noted that a cellular telephone used onboard an airborne aircraft would have greater range than a land-based handset, and its signal would be received by multiple terrestrial cell sites in a given market, causing harmful interference. Moreover, the Commission found that because a cellular telephone can transmit on all assigned 800 MHz cellular frequencies, a single handset could interfere with cellular systems in multiple cellular market areas simultaneously. Thus, the Commission concluded that “the need for noninterference in all cellular transmissions outweighs the benefits that would be realized by allowing the
  public to use cellular service in airborne aircraft.

The FCC is going to make cell phone calls allowed at altitude, but only in conjunction with airborne cell sites that would be located on the plane.

authors: Золбаяр haPpy


text:
Is there any particular reason for the cockpit of the 747 being on the upper deck?
The cockpit of the 747 is on the upper deck, but the cockpit of the A380 is situated on the lower deck. Is there any reason (be it engineering or anything else) for the location of the cockpit?

the A380 cockpit is above the lower deck, as it can be seen in this image, you have to walk up some steps to reach the cockpit level from the lower deck.

Um, how safe is the first seat row in the lower deck? On other throughts, if they could have made the nose of glass, it would have provided a great view from the lower deck.

The lower deck is the first-class cabin, and the seating in that cabin depends on the airline, but as one example, United's 747-400s have only 12 first-class seats on a flight. As far as safety, the first-class sections on most planes tend to have relatively low survivability in a crash; first class is where it is on planes primarily for convenience.

Boeing actually never expected the 747 to be so successful. They thought supersonic aircraft were the future. But Boeing knew that an aircraft the size of a 747 would be great for the cargo market. So they designed one aircraft that could be easiy converted into a cargo version.
The cargo 747s can literally open their nose. It is quite impressive. To accomodate this, the cockpit needed to be on the upper deck. Hence the passenger version had the same requirement.

Thank you @Ben, but does that mean Airbus does not intend to make a cargo version of the A380?

@MadhavSudarshan not necessarily it just means that loading will be through a standard cargo door in the side.

According to Wikipedia, Airbus does intend to market an A380F variant, but the passenger version is receiving the lion's share of orders, and Airbus is betting the VLA market (400+ seats or 130 tonnes) will deal primarily with passenger traffic between highly-congested airports so they're working full-tilt on the passenger variant. Boeing is thinking that cargo between those same airports will be a much bigger component of the VLA market, and its venerable 747 platform currently has no practical competition in the VLA cargo class.

@KeithS - The A380F was launched at the same time as the A380 passenger version. It initially received a few orders, from Fedex, UPS and ILFC, however as the A380 program began to experience problems prior to first flight, they scrapped the freighter so they could concentrate resources on getting the passenger version in the air.

The A380 wasn't particularly popular as a freighter, due to a few design constraints. First off, the split deck is structurally required, and cannot be removed. This limits the physical dimensions of freight that can be loaded. Additionally, the upper deck is not terribly strong, and cannot carry much weight. Therefore the A380F was only ever of use for parcel carriers, hence the orders being limited to UPS and Fedex.


text:
Fly a fixed wing single propeller airplane like a helicopter?
Has anyone tried to "helicopter" an airplane?  And succeeded?  And posted this video on the internet where I can find it?
What I'm talking about with what I would call a "helicopter" maneuver in an airplane is to fly with the nose pointed nearly straight up while maintaining a constant altitude.  I don't expect the airplane to hover, and I expect that to be impossible since the torque of the engine would leave the airplane spinning like a top.  I expect the airplane to move horizontally quite slowly, slowly enough that the wings provide very little lift.  With the nose of the aircraft pointed nearly vertically it should be quite obvious that nearly all the lift is from the prop.  If there's a different name for this maneuver then I'd like to know what that is.
What makes me ask is that I saw a battery powered remote control airplane that flew in this manner.  It looked like a typical remote controlled airplane except its single propeller appeared rather oversized for the airplane, and it had small hooks on the end of stiff wires that extended forward from the wings to out beyond the propeller.  The "trick" it could do was that it could hang from these hooks vertically from a horizontal wire suspended in the air and launch itself into the air using this single propeller for thrust.  It could then fly about like a typical airplane would.  Then to get back to it's launching point it would point its nose into the air and fly about almost like a helicopter.  As I recall it could steer itself around using control surfaces no different from those on any airplane, at least nothing stuck out as unusual about the control surfaces and I don't recall anything in the commentary that would indicate the control surfaces deviated from the norm.  It could travel horizontally "forward" (belly first), "backward" (top first), perhaps even side to side (wingtip first) while maintaining a consistent altitude.  It could navigate quite well this way with its nose in the air "helicoptering" about, then once it found the wire it launched from with its hooks it could cut the engine and hang from the wire.
The commentary on the video I saw of this aircraft explained that the idea was to use this as an observation drone that was capable of flying like any airplane would to get high speed and long range, at least faster and farther than one could expect from a quad rotor drone, as well as achieve a near hover, able to follow a slow moving target or make tight circles about a stationary target.  The idea of launching from and returning to a horizontal wire was to allow the drone to recharge from overhead power lines.  The aircraft's ability to launch or recharge from overhead power lines didn't interest me as much as the idea of an aircraft that looked like a very typical single propeller airplane flying around like a helicopter.
I can imagine that this is an impossible maneuver by most aircraft by the virtue of the propeller providing all the lift.  This would require an airplane with a thrust to weight ratio greater than 1-to-1, and likely other features like a radiator capable of dissipating the engine heat with far less than optimal airflow.  This would require a powerful airplane like those built for racing and aerobatics.
The question isn't so much about if this is physically possible.  This radio controlled airplane proved the aerodynamics worked.  I've seen full sized piloted propeller airplanes go straight up before, and reaching considerable speeds and altitude in the process, so there's propeller aircraft with enough power to pull this off.  The question is more like, is there video of full sized airplane with a pilot inside flying about like a helicopter?
I can imagine an airplane flying like this down to a runway and landing in a space not much bigger than the plane itself.  It could come in real slow down to the runway and then shortly after the tail wheel touches the ground it "belly flops" on to the front wheels.  With all wheels on the ground the prop could be feathered and brakes applied to minimize forward movement.
Then for an encore the airplane could tilted up with some kind of hydraulic ramp to nearly vertical, and take off almost straight up into the air.  I wouldn't expect it to zoom off like some rocket, or even burst into the air like that little electric model airplane.  I would expect more like it slowly lifting itself from the ramp, likely twist about slowly from the prop torque to clear the ramp, then get a bit of forward motion to steady itself, and then gain some altitude and transition to level flight. Not only would this take a high performance aircraft but a high performance pilot.  I suspect it would be very dangerous to get close to the ground flying like this.  It would be dangerous to do high in the air, but at least with some altitude and a powerful enough engine recovery from a screwup is possible.
I recall experimental "tail sitter" aircraft with counter rotating propellers built specifically for vertical takeoff and landing.  That's not what I'm asking about.  I'm asking about a single engine, single prop, and largely "off the shelf" airplanes flown in a manner where all the lift is from the propeller and it's flying with no change in altitude.
Has anyone flown an airplane about like a helicopter?  Is there video evidence of this?  Is there a term for this kind of flying other than "helicoptering"?  If this is considered impossible, as opposed to merely suicidal, then I'd like to know why.

Some aerobatics planes can do this. https://www.youtube.com/watch?v=R_EMX7N9PjA

Yes, fast forward to about 3:10 to see the vertical hover in that video.

@GregHewgill  That proves there are single propeller airplanes with sufficient power to maintain a consistent altitude but it does not show the airplane maneuvering horizontally.  Perhaps the near comically large propeller I saw on the remote controlled airplane example is necessary to get enough air moving over the wings that the control surfaces can be used to move the airplane about.  With that biplane example the control surfaces would likely be far enough out on the wing to not catch any airflow from the propeller.

See this post https://aviation.stackexchange.com/questions/65590/is-this-video-real/65593#65593  Wayne Handley and his PT-6 powered Raven is the only time I've seen it done with the airplane in complete control with the ability to maneuver at will.  He stopped doing the act in the late 90s, I believe because it was just too dangerous close to the ground.

@JohnK  An excellent answer.  I'm having trouble finding video of this feat.  It's not that I don't believe you, I'd just like to see how well it was done.  I'll consider this an answered question once I see some video.

I never could find a video of his act from those days, so you're out of luck I'm afraid.

Broadly speaking flying a single engine prop plane like a helicopter is not really possible since airplanes lack the swashplate and cyclic style controls that make a helicopter practically maneuverable and at the low vertical airspeeds this maneuver would occur Im not sure how effective the control surfaces would be. With sufficient thrust you likely could get a prop plane to take off from a vertical position but I dont know of any light singles that have that kind of performance (even fancy jets only governments can afford hover just over the 1:1 ratio).

The question was specifically about single propeller aircraft, the Osprey and Pogo do not qualify.  I stated an understanding that this would not be something a general aviation airplane would be capable of doing but rather in the realm of a plane built for racing and/or aerobatics.  Given that this was demonstrated with remote control aircraft there is ample evidence that with the control surfaces close enough to the root of the wing they can get enough air movement from the propeller to be effective in maneuvering the airplane while in a hover or slow moving near hover.

I see you edited your answer to remove the mention of dual prop aircraft.  I was not asking about an airplane flying precisely like a helicopter with cyclic controls, just generally with the ability to move about slowly in a near hover.  I am fully aware that this is not in the realm of most aircraft, I stated as such in my question.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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What maneuvers can a non-pilot perform when flying with a private pilot?
I recently got my private pilot license and will soon be taking people for rides. My son wants to get his license (he passed his written test), so I was thinking I could introduce him so basic maneuvers. As a pilot, but not an instructor, what can I safely let a non-pilot do? Obviously anything in the pattern will require myself to be in control, but is it OK for a non-pilot to do slow flight or steep turns? Or would it be better to just perform them myself and let them see how its done? As pilot in command, it comes down to what I feel is safe, but are there any regulations that prevents a passenger from taking the controls and doing some turns? I don't remember this being covered in ground school or any of the training books.

See also Are pilots allowed to let passengers fly the plane?

Yeah, I saw that, but this question is more in line with what training maneuvers can a non-instructor allow a non-pilot to perform. Are there any regs that limit what a pilot can let their passenger can do? Suppose someone has an aerobatic plane, can a non-instructor allow a non-pilot do loops?

This question is really a dupe of the other one: whether you're 'training' your son or not is irrelevant, you're still the pilot and he's still a passenger, so it's up to you to determine what's safe and what isn't. Since you just got your PPL yourself, you might want to consider if you can demonstrate maneuvers effectively at all. You might inadvertently introduce him to some bad habits and - unless you're a teacher yourself - you haven't been trained to teach. You may also not have the experience/training to recognize a potentially dangerous situation developing.

Is the question about regulations in some locale or about what can safely be done? You might want to clarify the question and highlight how you think it differs from the previous one.

Old topic, but I'd say he can do pretty much anything not requiring a parachute, and probably best to stay out of airspace where radio work is required to start.  As you're not a CFI, he can't log any of the hours as Dual Received either.

Since this is already answered, you can view there, but I'll say this out of concern for your safety - while the PIC can legally let a non-pilot fly, this is something that you should ease into as a low time pilot. You should let a non pilot do maneuvers like slow flight or steep turns, as they are potentially dangerous maneuvers, and as a low time private pilot, you do not have the experience to reliably account for every mistake they could make. You could alternatively do them yourself, but allow your son to follow the controls.


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What special airspace exemptions are granted for civilian fighter jets?
How do civilian fighter jet operations arrange airspace usage for conducting flight tours? Do they use military operation areas? If they fly into class A can they perform maneuvers, or do they need to stay below class A? One such operation is migflug.
When flying in airliners cross country I look out the window and think what it would be like to experience flying around and between cumulonimbus clouds in a jet fighter, but doubt ATC would allow it. It got me to wondering how the civilian jet tours handle airspace and speed restrictions.

Civilian aircraft are perfectly free to use MOAs at all times. There's no special arrangement needed; MOAs are regular airspace unless the military declares them active, at which point the military can do their own thing (which you can still freely fly through, they'd just rather you didn't because they might have to pause training and fighter time is expensive).

All civilian aircraft, regardless of their origin, are subject to the same restrictions and regulations.  The only variations are to take into account the performance of the aircraft (e.g. you don't slow a 747 to 120kts on approach because there is a slower aircraft ahead).  The fact is it a fighter means nothing.

If they are in class A, can civilian fighter jets can do anything they want, loops, etc, as long as they are on instrument flight rules and in contact with ATC?

The same regulations apply to civilian ex-military aircraft as they apply to any other aircraft. Whether flying IFR or VFR, you will need to comply with the same rules, e.g. speed restrictions, airspace restrictions when under VFR, meteorological conditions, etc.
An exception can be speed restrictions, where a speed restriction would be in contradiction with the parameters of the aircraft to perform safe flight, e.g. the minimum clean speed being above 250kts or similar.

As an additional FYI, fixed-wing tactical aircraft that find their way into civilian hands are usually registered as Experimental due to their being unable to meet certain certification criteria under Parts 23 and/or 25. The end result is that they may be subject to certain additional restrictions on a case-by-case basis.

@habu: What sorts of certification criteria can't they meet?

@Sean you'd need to go through the entirety of Parts 23 and 25 to cover everything, but the one that immediately comes to mind is the 61-knot stall speed requirement for single engine aircraft. Meeting this criteria would be impossible even for high-performance WWII straight-wing piston-engined aircraft, nevermind something like a Cold War-era MiG-21 (I once saw one listed in Trade-A-Plane for low hundred k's USD)


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What is the purpose of an inboard aileron?
I've noticed that larger Boeing aircrafts (such as the 777, 767, 747, etc) are equipped with inboard ailerons. What is their purpose and why hasn't Airbus incorporated them on the A330 or the A340?

the question is actually having 2 parts. The need of inboard aileron (duplicate) and why Airbus' designs are not including it (new one). I will propose to not close the question but reformulate it making the link to the answer to the first question

Airbus aircraft have no inboard ailerons because they move the flaps slightly differently: Whereas Boeing aircraft move them perpendicular to the hinge line, Airbus planes move them straight back. This avoids any interference between flaps on wing sections of different trailing edge sweep and the need for filling the gap with a flaperon.

As far as I know, the inboard ailerons are used for high speed maneuvering.
The rolling moment induced by the ailerons is linearly dependent with the distance to the center of gravity (e.g. the farther out, the stronger the moment, the harder the roll).
As the aileron is an aerodynamic surface, the force induces is equal to:
$$F_{aileron} = c_l(\alpha) \frac{1}{2} \rho V^2 S_{aileron} $$
The moment induced is equal to this force, multiplied by the distance to the c.o.g.:
$$M_{aileron} = y_{aileron} \cdot c_l(\alpha) \frac{1}{2} \rho V^2 S_{aileron} $$
If you're flying in cruise, and you want to make a smooth turn, you need a relatively small $M_{aileron}$.
However, the $V^2$ term, is already very large. So we want to make the rest of the terms relatively small.
We could do this with outboard ailerons: Use the outboard aileron with a large $y_{aileron}$, combined with a small $c_l(\alpha)$, usually the outboard $S_{aileron}$ is fixed by the low speed control requirements.
However, you can provide $M_{aileron}$ more accurate if you use the inboard ailerons (with a small $y _{aileron}$). The smaller distance makes the moment a bit smaller, and less susceptible to small deviations.
I think the Airbus A330 & A340 use their spoilers to maneuver at high speeds. They simply extend their spoilers -which are usually also reasonably close to the c.o.g.- a tiny bit, this changes the lift distribution, also causing a rolling moment. I did not research this fact, so I'm not 100% sure this is the reason the A330 and A340 do not have inboard ailerons, but I do know some aircraft use this method for high speed maneuvering.

As I remember the outboard ailerons in the 727-100 and the 747-100 and -200 are locked out (not used) when the trailing edge flaps are up.


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Are there ground stations at the waypoints a pilot may fly to?
How is a waypoint marked? Is there a ground station present at that waypoint? And how are these waypoints named?

The naming part has been answered already

Ground based radio navigation (VOR, VORTAC, LORAN, NDB) by necessity have ground stations to handle generation, amplification and broadcasting the signals used for navigation.  Here is an example of what a VOR station looks like:

"Table Rock VOR" by ZabMilenko - Own work. Licensed under CC BY 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Table_Rock_VOR.jpg#/media/File:Table_Rock_VOR.jpg
Waypoints that represent these stations will have ground facilities underneath them, but they are the minority of waypoints.  Most waypoints are defined as a distance along a radial from one of these stations, so there will be no ground station below them.  Other waypoints are lat/lon fixes provided by GPS, so there are no stations below them.
Keep in mind, no matter if there is a ground station or not below a waypoint, their presence is not important to how navigation is performed.  Waypoint passage will be noted by needles, to/from indicators, DME mileage changing, or by a points along a magenta line on a map display.

VOR stations are often named by their location e.g. the VORTAC in San Antonio, TX is named "San Antonio (SAT)".  Others are named for regional things, e.g. "Ranger (FUZ)", "Cowboy (CVE)" and "Maverick (TTT)" all located in Dallas, TX named after their major sports teams (baseball, football and basketball respectively)


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What is the point of making one control surface play the part of multiple control surfaces?
For example, flaperons (flaps + ailerons) and elevons (elevator + ailerons), are all individual control surfaces but play the roles of two different control surfaces. What is the point of making a single control surface, undertake the functions of two?

Spoilerons on the MU-2!

This is speculation, so give it the credibility it deserves....
If the required aircraft performance dictates ailerons to be essentially the full length of the wing (as in the plane you pictured), there may not be sufficient room for flaps to also be as effective as needed.  So the solution is one control surface long enough to satisfy both needs.

The Extra 300 seems like an example of this.

Flaps which only extend over part of the wing will cause lift changes mainly over that part of the wing, which will result in a lift distribution over span which is far from the elliptic ideal once the flap is deflected. Therefore, it makes sense to move the ailerons in the same direction, especially when the inboard flaps are simple camber-changing flaps.
In order to give an aircraft wing docile stall characteristics, the planform should not be elliptic, but be "fuller" (= having more local chord than the ideal ellipse) at the wingtips. With an elliptic planform and an elliptic lift distribution, the local lift coefficient will be identical over the whole wing, and gusts might easily cause a local stall at one outer wing in slow flight. The consequence will be an uncontrollable roll, which can ruin the pilot's day when occurring close to the ground. Therefore, it is better to leave some margin by increasing local chord and adding some washout towards the tips.
Now this wing will have an elliptic distribution only at one angle of attack. If the angle of attack is higher, the lift distribution will be fuller, and if it is lower than the "elliptic" angle of attack, some lift will be missing towards the tips. Both conditions will cause higher induced drag than necessary. Flaps help by adjusting local camber such that if they are set correctly, the spanwise lift distribution will be much closer to the elliptic ideal than without flaps. This requires the flap schedule to make bigger deflection changes towards the root - watch any modern high performance glider change flap settings, and you will see what I mean: The flaperons will change deflection less that the inboard flaps. This has the additional benefit of leaving more margin for aileron deflections.
This is the disadvantage of combined control surfaces: What is needed for one function is no longer available for the other function, because the maximum useable deflection range is limited.
For the nitpickers: Please understand the term "elliptic lift distribution" as "elliptic circulation distribution". Then all is well.

For commercial flight, I think the biggest drawback is safety. With combined control surfaces, such as flaps/ailerons = flaperons, a failure in the actuator results in a failure of 2 controls instead of 1. In addition, there will be less efficiency from a torque-per-drag standpoint, because usually the combined surface is not optimally shaped for either role. But it saves weight by collectivizing things, so there could be a recouping of efficiency. That is why I said the biggest drawback is safety (for commercial flight).

Several good reasons depending on the design and intent of the aircraft:

Mechanical simplicity. In your case of "flaperons", having a single set of control surfaces do two things decreases the number of moving parts the plane has. Simple is best; the simpler your aircraft, the less can go wrong (and the lighter it is, and when you're trying to get something off the ground, lightweight is good too).
Durability. Even when mechanical simplicity isn't as much of a concern and the craft has distinct flaps and ailerons (in multiple sections), making the ailerons react to flap position as well as stick/yoke input can help normalize the forces across the wing, reducing shear moments along the wing spar and other stressed sections of the wing, decreasing the potential for fatigue and failure. Some commercial airliners, particularly Boeing designs, have rear-wing control surfaces in three sections; innermost are a set of multi-tier flaps, then in the middle are "inboard ailerons" which act as ailerons at cruise speed but flaps at takeoff/landing, then the "outboard ailerons" which are used only at low speed.
Aerodynamics. Every square inch of surface area of the plane increases drag. Designs like the V-tail reduce total surface area by eliminating an entire control surface and stabilizer from the empennage, using the remaining two as "rudder-vators". This Beechcraft V35 is known as a very efficient cruiser especially at altitude (unfortunately they also have a bad rap for squirrelly roll behavior and a series of structural failures early in production, leading to their moniker of "Doctor Killers"):

Stealth. The YF-23, which lost the ATF competition to what would become the F-22 Raptor, used a similar rudder-vator configuration because it reduced the plane's radar cross-section compared to the 22's parallel elevators. The resulting plane was actually stealthier (and faster) than the Raptor, but less agile.

The B-2 Spirit, obviously, has no tail at all, so the primary control surfaces do quadruple duty as ailerons, elevators, spoilers/air brakes, and rudder (by opening the air brake on one side or the other). The craft has separate flap systems, so the primary control surfaces are only "spoilaileruddervators":

Computer control. The F-16 was designed with "elevons" instead of traditional aileron/elevator controls for a number of reasons, one of the primary ones being to reduce the required complexity of the F-16A's original analog FLCC. The only control surfaces on the wings themselves are trailing-edge flaps which are also under FLCC control and extended in any high-AOA situation. Delta-wing aircraft without canards, like the Mirage 2000, use elevons by necessity (the Rafale and Eurofighter use canard elevators but supplement them at high speeds or AOAs with the elevons).

Which airliner uses the split flap ailerons you describe?

I remember a few Southwest 737s  with what looked like flap sections extended under the ailerons. I'm probably mistaken as I can't find a single example of it; most references to flaperons on commercial airliners refer to "inboard ailerons", a control surface that responds as an aileron at high speeds (with the "outboard ailerons" locked out because they stress the wing too much at cruise) but a flap at low speeds; these are apparently fairly common on Boeing designs.

How does the B-2 deploy its flaps without pitching over uncontrollably?

@Sean - Very good question, which I'd like to know the answer to as well; I'd ask this as a new question.

@KeithS: Question asked.

@KeithS: Turns out they aren't flaps at all, so might want to edit your answer.

+1 for "spoilaileruddervators!"

Further speculation: Fewer control surfaces overall means fewer moving parts, less mechanical complexity, less weight, possibly less drag, and so forth.
The downside is that the control system must allow the combined control surfaces to be controlled in a way that makes sense to a pilot. This can be done mechanically, but especially in aircraft that are fly-by-wire anyway, combining the inputs in software has zero weight or mechanical-complexity cost.

Don't underestimate the complexity of a mechanical mixer. And using shorter sections of flaps with their individual links instead of one wide flap will raise the flutter speed noticeably.

Glider designers use flaperons frequently because the chord is so narrow and the wingspan so long that the ailerons have to span the length of the wing to have sufficient roll authority. This leaves no room to add flaps, so they are incorporated in to the ailerons.


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What is the purpose of having contra-rotating props on an aircraft?
Why do planes, like the famous XF-11 and the Tu-95, have contra-rotating props? What is their purpose?

Image credit: Mike Freer / Touchdown-Aviation / Wikimedia

These are called co-axial rotors in helis

According to my experience, you have two problems with props:

Torque
Exhaust Speed

If you use contra-rotating propellers, you eliminate the torque reaction problem because you now have two contra-rotating propellers each cancelling out the opposite's torque.
Also you increase the speed of air accelerated by the propellers, thus the aircraft’s efficiency at relatively high travelling speeds is increased.
Normally, the propeller can only be operated effectively at low Mach numbers and increasing the RPM above a certain point will only cause the air behind the propeller to compress but not accelerate. Using a second propeller with higher pitch angles can generate high enough airspeed without the problem of compression because the second propeller is rotating inside the airstream generated by the first propeller.
Correction
However, having a different speed and pitch WILL induce a torque effect.

This leaves out the 3rd reason: because it just looks so darn cool!

Increasing speed of the accelerated air decreases efficiency. Because higher $\Delta v$ means the air carries away more kinetic energy ($E = \frac{1}{2}mv^2$) for the same momentum ($p = mv$) and therefore you need more power ($P = \frac{E}{t}$) for the same thrust ($F = \frac{p}{t}$).

Contra-rotating propellers may have higher efficiency than equivalent (same diameter and same $\Delta v$) single propeller, but that is not due to increase in speed of the air.

Why would torque be an issue in a 4 engine plane?

The main advantage of contra-rotating props is the increased efficiency (6 to 16% according to Wikipedia). Another advantage is that the torque is pretty much cancelled out by the contra-rotating prop, but this is much less of an issue in aircraft with an engine configuration like the Tu-95 in your post.
I would say, another advantage is the fact that this design allows for a smaller propeller diameter, allowing a smaller landing gear, etc etc.
The disadvantages may outweigh the advantages depending on the design. They are very complex. This may lead to high maintenance costs, or an unreliable design. Contra-rotating props are also extremely loud. The Tu-95 is known to be the loudest aircraft in the world. Rumors are that they can actually be heard by submarines.
The fact that there aren't many aircraft using this design is proof that it isn't very popular. As far as I know, the only reason the Tu-95 doesn't use regular turbine engines is because there were no turbine engines available that would meet the required specification during the design stage of the aircraft.

Just a nitpick (and welcome to Aviation.SE!), but the turboprops on the Tu-95 Bear are turbine engines. What you are talking about are turbojets or turbofans!

From what i heard submarine can hear most aircraft flying at low altitude anyway.

@vasin1987: I'm talking about a Tu-95 flying at cruise altitude.

The extra efficiency gain is because an isolated propellor expends some energy imparting a "swirl" velocity to the air (i.e. an angular momentum), in addition to pushing it backwards (a linear momentum) to cause thrust.  The second contra-rotating propellor operates opposite to the swirl direction of the first rotor, resulting in a low net-swirl and recovering the energy wasted in turning the flow while still contributing to the rearward momentum that causes thrust.

As I understand it, the TU95's noise comes not so much from contra rotating propellors, but from the prop tips going supersonic. There was a turboprop version of the F84 Thunderstreak that had a supersonic prop. So noisy, that it was nicknamed Thunderscreech.

While there are theoretical benefits in terms of efficiency, the difficulties in gearing them have kept them from practical use except when used with very high powered engines.  After a certain point, using longer blades becomes increasingly inefficient, and other strategies have to be used to increase the swept volume.  An example would be the Spitfire, which went from 3 blades in early marks, to 4, 5, then a contrarotating 3+3 in the Mark XIX as the power of the engines increased over the production run.
Also, it is worth noting that virtually all turbines have many compressor stages which turn on one shaft but have static blades (stators) between sets which provide the benefit of reducing the tangential flow without requiring high speed gearing.  This provides nearly all the efficiency increase without adding the mechanical complexity of trying to spin each stage in opposite directions at the high RPMs of a turbine.

Apart from the reasons stated above, there is another important one, specially valid for the Tu-95. The turbines of that plane are very powerful, the most powerful ever built. In order to convert that power in acceleration of a mass of air, you would need a very large and fast-spinning propeller, so large that the blade tips would surpass the velocity of sound. Thus, the only possible compromise for the giant turboprops of the Tu-95 was to fit each turbine with two large coaxial propellers, spinning in opposite directions. Even with this solution, the blade tips are supersonic, but not by much, and the propeller efficiency is good enough to propel the Tu-95 to speeds only slightly lower than its American jet-propelled counterpart, the B-52. The Tu-95, still flying today in the air forces of Russia, was a great technical success of the Soviet technicians.

Why do these planes have four sets of counter-rotating props? Not torque compensation - they have four engines and can simply zero out torque by having two turning clockwise and two anti- clockwise.
Counter-rotating props on fixed wing aircraft are a way to overcome the inherent limitations that propellors have in converting engine power into thrust. Propellor tip speed is the issue, once it reaches speed of sound a lot of engine power is required to overcome compressibility drag, and not useful in propelling the aircraft. So the disk size is limited, or the rotation speed of the blade. More blades help a bit if power is not the issue, but only a bit. More propellor disks help, but again only to a limit.
The Tu-94 had a similar operational mission to modern passenger jets: fly long range, close to the speed of sound. It first flew in 1952, a long time before high bypass turbofans existed: the jets of the time were wasteful with fuel, also a consideration in the design of the B52. While the US was working on more efficient turbofans, the USSR chose the other option: use two propellor disks. The second disk further accelerates air from the first disk.
New user Sam posts: Seems like we need to take a look at the potential better efficiency of wind turbines. Indeed a counter rotating arrangement can extract more power from a given air flow, but at what cost? Is tip speed breaking the sound barrier really an issue with wind turbines? Does the counter-rotating arrangement cost less than simply constructing another complete generator?

Contra rotating props probably wouldn't matter for windmills. Airspeed is low, and you'll get more juice by putting two props adjacent to each other running two generators, rather than two props rotating in the same space, where the first prop has already extracted a good deal of the energy in the air flow.

Indeed, my questions are self-answering. Theoretically possible but of no practical or economical value.

“ Why do these planes have four sets of counter-rotating props? Not torque compensation - they have four engines and can simply zero out torque by having two turning clockwise and two anti- clockwise.” because when you’re dealing with an engine which can produce 15,000 shp, it imposes huge axial and torque loads on the engine mount and wing structures.  It would be nice to eliminate that torque load and I’m guessing that was the thinking at the Tupelov Bureau when they designed that thing.

Here is an explanation of a contra-rotating propeller, explained its advantage and disadvantage.
2 - Why contra rotating propellers?
As explained, each blade of an airplane propeller is essentially a rotating wing. As a result, the propeller blades are like airfoils and produce forces that create the thrust to pull, or push, the airplane through the air.
The high-speed rotation of an airplane propeller gives a corkscrew or spiraling rotation to the slipstream. The corkscrew flow of the slipstream also causes a rolling moment around the longitudinal (roll) axis. Note that this rolling moment caused by the corkscrew flow of the slipstream is to the right, while the rolling moment caused by torque reaction is to the left—in effect one may be counteracting the other. {However, these forces vary greatly and it is up to the pilot to apply proper correction action by use of the flight controls at all times. These forces must be counteracted regardless of which is the most prominent at the time.}
At high propeller speeds and low forward speed (as in the takeoffs and approaches to power on stalls), this spiraling rotation is very compact and exerts a strong side-ward force on the airplane’s vertical tail surface.
When this spiraling slipstream strikes the vertical fin on the left, it causes a left turning moment about the airplane’s vertical (yaw) axis. The more compact the spiral, the more prominent this force is. As the forward speed increases, however, the spiral elongates and becomes less effective.
One solution is by adding a second propeller behind the first, contra rotating and canceling the rotating moment.

Simple answer - To counteract Torque effect.
Detailed answer - Propellers usually rotate in the clockwise direciton. Now recollect Newton's 3rd law - "Every action has an equal an opposite reaction". Here the "action" is the propeller rotating clockwise and the "opposite reaction" would be the aircraft having a tendency to turn in the anticlockwise direction (that is, towards left). Now this left turning tendency is going to be a pain for the pilots as they have to apply rudder pressure to counteract it. And that's why contra-rotating propellers were introduced!
As the name suggests it's basically 2 propellers rotating in 2 opposite directions (one would be clockwise and the other - anticlockwise). This is going to cancel out the torque effect!

Regarding noise of propellers at speeds close to sound barrier, you may have a look at the F-84 'Thunderstreak', propeller version video in YouTube, https://youtu.be/UFhSzReWTgs it has contra-rotating propellers, and is probably one of the fastest airplanes with this arrangement ever, the video discusses the development and findings of a contra-rotating propeller in a fighter flying close to Mach 1; at any speed, the aim of having contra-rotating propellers I guess is reducing the effect of torque that will tend to make the airplane turn in the opposite direction as the propeller, along the propellers axis, in a centrally placed, single engine airplane, a propeller rotating in a clockwise direction, looking from behind, will indice a force making the fuselage turning in the opposite direction, also the vortex and other changes in air column induced by the propeller may have an influence on control elements, such as the rudder

i'm not sure this has anything to do with the question posed.

This airplane, F-84 propeller version, had contrarrotating propellers, and is an special experience in the field, because of the speed it flew at, some connection to contrarrotating propellers, it could be considered it has.Thanks. Salut +

it would be useful if you could [edit] your answer to include that information.


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Why do turbofans have different nozzles?
The CFM 56-7BE (on the 737) has a long and narrow nozzle, the RR Trent 900 (on the A380) has a sharper nozzle, and the CF-6 (on the 767) has a wider opening at the nozzle. Why do the nozzle sizes vary among turbofans?

What do you exactly mean by nozzle? Are you talking about nacelle design, nacelle inlet or nacelle exhaust?

I think the question is about nacelle exhaust section, but pictures illustrating exactly what you are asking about would be helpful.

Thank you @Farhan for correcting me, I meant the nacelle exhaust.

If you can add pictures ilustrating exactly which differences are you looking for be explained will allow us to provide you a better answer.

The nozzle, apart from its main function, that is accelerating the air in order to propel the vehicle, it determines what goes into the front of the engine, it is a downstream restrictor along with the turbine nozzle.
The areas of both the propelling nozzle and turbine nozzle set the mass flow through the engine and maximum pressure. For different engine operational purposes, it is used one shape or another. These engines have different shapes due to optimization reasons; the engine, is designed in such a way that the aircraft features will have the best balance in terms of fuel consumption, thrust needed, and many other parameters.
Each manufacturer design their engines in the most convenient way and then, it is determined which engine is better for a given aircraft.
Different kind of geometries, (e.g. convergent-divergent) also affect on the engine operation regime (supersonic, subsonic, transonic), but this does not affect here since all the engines are turbofans and thus, subsonic.

Depending on what the question is referring to, the part may not be made by the engine manufacturer.

The nozzle shape also has quite an effect on engine noise. Some older engines have been equipped with "hush kits", which change the shaping a lot.


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Is there an explanation for the measurement system used in aviation?
Why is speed measured in knots (as opposed to kmp/h or mp/h), and distances in nautical miles? Is there a reason for following such a system?

But this isn't standard around the world.

@Zizouz212 but it is predominant!

Note: That question does not directly ask for reasons, but the answers there mention them.

To quote from the Wikipedia article on the subject:

Although the unit knot does not fit within the SI system, its
  retention for nautical and aviation use is important because standard
  nautical charts are on the Mercator projection and the scale varies
  with latitude. On a chart of the North Atlantic, the scale varies by a
  factor of two from Florida to Greenland. A single graphic scale, of
  the sort on many maps, would therefore be useless on such a chart.
  Since the length of a nautical mile, for practical purposes, is
  equivalent to about a minute of latitude, a distance in nautical miles
  on a chart can easily be measured by using dividers and the latitude
  scales on the sides of the chart. Recent British Admiralty charts have
  a latitude scale down the middle to make this even easier.[8]

e.g. BA Chart 73, Puerto de Huelva and Approaches, 2002

It's always nice to link to what you're quoting, especially when it's your entire answer!

Knots are a measurement of nautical miles per hour. As long as we are using nautical miles for distance for navigation purposes, it would be silly to use anything other than nautical miles per hour.
One nautical mile is one minute of latitude. Maps can therefore provide a pretty accurate measurement of distance even at the higher latitudes as long as there is a lat/long scale. If you were using anything but nautical miles as your measurement, you would have to add an extra step in to convert.
Finally, if it 'aint broke, don't fix it. The current system certainly isnt consistent from country to country. But if you were to change things, the likelihood of everyone agreeing to using the metric or imperial system? It's much easier to stick with this neutral measurement.


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Is the autopilot system capable of taking off and landing a plane?
I am aware of the fact that the autopilot is engaged after takeoff and disengaged before final approach but I was just wondering whether or not the autopilot is capable of landing or taking off a plane.

For landing, see Which commercial aircraft are capable of computer-only landings, without human assistance?

depends, which "autopilot system" of the several existing are we speaking of?

Not familiar with ANY autopilot that is ever used for takeoff with a manned aircraft. Some autopilots are capable of auto-land, although the requirements for the compatible systems on the ground are quite strict.

@RalphJ for now :P

No and there is no point in auto takeoff because pilots are trained to do this themselves and if technology fails on an aircraft the pilot wouldn't know what to do and if there was a such thing as autotakeoff and landing pilots would count on technology to do the work for them. I know this is an old question but this is what I do when I am bored on this website


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What are the protrusions on the sides of the fuselage of the F-16I?
I've noticed that the F-16I has these protrusions on the sides of its fuselage. What purpose do they serve?

They are Conformal Fuel Tanks.
See http://israeli-weapons.com/weapons/aircraft/f-16i/F-16I.html:

Conformal Fuel Tanks (CFT) - These tanks are manufactured by the  "Israel Aircraft Industries"  and increase the aircraft's amount of internal fuel by 50%. Their purpose is to significantly prolong the aircraft's flight range and ability to remain in the air. The tanks are installed the plane's back and conform to it's figure.

Israel does not have any heavy bombers, so they need the fighters to be capable of striking deep in enemy territory and since they can't be accompanied by tankers there, they need additional fuel.
As jwenting mentioned in comment, unlike most conformal tanks that are removable on the ground, these are fitted permanently.

Thank you for the informative answer. But why not have external fuel tanks mounted on the lower surface of the wing?

@MadhavSudarshan: In the picture, the two large bodies on inboard wing hardpoints and the one on middle ventral hardpoint are also external fuel tanks. They want even more fuel then that. Because they plan missions which USA would use B-2 or B-1B for, but Israel does not have those, so they have to do with F-16 and as much additional tanks as they can manage to add.

mind that the conformal tanks on the F-16I are permanently fitted, the blisters also containing extra equipment bays, not just fuel cells.

Without benefit of reference to their flight manual; I'd also say that hard-point-hung tanks restrict roll rates and G loading vis-a-vis the conformal tanks. Alternatively, more room for ordinance.

Although you call them permanently fitted, they might be that, but they are removable.


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Why is it that only light aircraft are equipped with airframe parachutes?
Why is it that only light aircraft (like Cessnas and Cirrus') are equipped with airframe parachutes? Isn't it possible for at least slightly larger aircraft to have this feature? And isn't there a way to recover the fuselage of a relatively larger aircraft?

@Federico, while that question talks about much larger aircraft, and this one specifies slightly larger aircraft, I'd agree that most of the reasoning there is applicable.

@FreeMan then there is this one: Why don't all new GA planes include a parachute system?

FYI the situations in which an airframe parachute would be useful are very rare and the probability does not justify the loss in cargo capacity.

Bah! RedGrittyBrick got in on the voting and messed up our alliteration in F!  :/


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How does a pilot board a B-2 Spirit?
I haven't noticed any sort of plug door, so how do pilots board the B-2?

There is a hatch and a ladder in the bottom of the front fuselage, through which the pilots enter the bomber, as seen here:

Why have such a design? In the case of a belly landing, wouldn't the pilot be trapped inside?

@MadhavSudarshan I would imagine it has something to do with the process of applying various stealth technologies and materials on the top of the fuselage, which something like a movable canopy, for example, could have compromised. Also, considering how the United States have superiority when it comes to advanced stealth technologies, I assume that an emergency landing on hostile territory would necessitate a self-destruction of the bomber. On friendly territory the plane could just be taken apart to get the pilots out; the bomber would be ruined from a stealth flight perspective anyhow.

@MadhavSudarshan there are most probably zero-zero ejection seats and the upper escape panels above the cockpit might be able to jettison on their own.

@MikeFoxtrot - there are indeed ejection seats, as the only loss of a B-2 did not involve the pilots - they ejected safely. In most ejection seat aircraft, ejection is preferable to a belly landing

There are still ways to escape if the main exit is blocked. A B-1 has a similar arrangement, and the pilots got out after a belly landing.


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What would happen if a pilot tried to land with a tailwind?
What if a pilot was forced to land in a tailwind? Would this be possible and what is the maximum tailwind an aircraft can handle?

Possible duplicate: Why is tailwind during final approach and landing so dangerous?

Your landing rollout will be longer, your groundspeed will be faster and you will stress the brakes more than a landing with less tailwind.  The same can be said for choosing calm winds over a headwind.
The jet I flew and many others have a limitation for maximum tailwind of 10 kts and it isn't unheard of to request an opposite direction landing (e.g. with a tailwind) if the tailwind is acceptable, the runway is long, and there is no conflicting traffic when it is convenient for the operation.
If you were forced to make a landing with a tailwind exceeding your limitations then the situation is probably an emergency and you'll want a longer than normal runway.  There is an increased threat of a brake fire, but in an emergency ARFF will be following you down the runway and they'll take care of it.  The tailwind landing only really becomes a problem if the runway isn't long enough (overrun) or if the tailwind is so strong that the maximum wheel speed is exceeded (loss of directional control or worse).


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Why are there two IRS switches on the 737NG?
I've noticed in some videos,that there are two IRS switches on the 737NG. Why is that so and what is the primary purpose of IRS? Also, what is the alignment of the IRS mean?

There are only two IRS on 737ng? Most aircraft have three of them.

@JanHudec Yep, "left" and "right".  I don't know why left and right rather than one or two.

It's there to ensure you file your tax return correctly :)

You know what I mean @vasin1987

Inertial Reference System is a solid state set of 3 ring gyros which navigate using dead reckoning.  The system measures accelerations in  and rotation around each of the 3 axis so can calculate where it is by adding how much it has moved to the initial position.
A simple example:
The aircraft is stationary pointing North.  Over a 10 minute period, it measures the accelerations in the X and Y axis and calculates that is has moved +100 kms in the X axis (i.e. North) and +100 kms in the Y axis (i.e. West).  By simple trigonometry, we can calculate that the aircraft is approximately 141.4 kms on a bearing of 45 degrees from it's initial position.
The gyros must be aligned with the aircraft frame of reference when it is stationary.  This is the "alignment" process and generally must be completed before the aircraft moves.  It may also take inputs of the current position from a manual keyboard or from some other system, e.g. GPS.
More modern IRS fits can align when the aircraft is moving, taking an updated position from GPS.
There are 2 systems on the 737, simply for redundancy.
This slide show gives a good overview.

@MadhavSudarshan You're welcome.

Is this alignement a one time event or must this be done regularly?

IRS panel
There are 2 selectors, one for each unit. There are 2 units for redundancy and certification.

(source)
Purpose of the IRS
The IRS (inertial reference system) function is to provide the aircraft location/attitude in flight, computed from the position/attitude sensed during its alignment and rotation/acceleration variations accumulated since the alignment.
An IRS provides a reference to the true north (and often the magnetic north equivalence by reading in a table).
In airliners, inertial references are generally included in a broader computing chain, the ADIRU, which also provides air references calculated from air probes (a.g. airspeed and angle of attack).
How it works
An IRS is composed of 3 gyro-lasers (while INS are based on mechanical gyroscopes) to sense rotation movements, and of 3 accelerometers to detect linear accelerations.

(source)
Alignment
The purpose of the IRS alignment is to detect the starting latitude and attitude, by sensing the Earth acceleration. It must be done when the a/c is at rest. To minimize the sensor error, the values sensed are integrated over a period of time (10 mn or so).
The longitude which cannot be sensed using Earth acceleration is provided through other means (like GPS or last position).

(source)
Attitude only mode
If the IRS is restarted in flight, then only the attitude can be sensed again. The location reference will be lost (this is what means the ATT mode on the selectors).
Use of IRS
IRS are a complement of the GPS, but IRS has the advantage of being independent of any ground or space equipment. This is why there are still found in the avionics of modern aircraft.
Accuracy
Sensing and integration errors accumulates with time after the alignment.
Precision of IRS compared to other systems:

(source)

I read through the linked source page for the IRS Display image and didn't see anything... Any idea what are the extra segments in the left-hand digits of the numeric displays at the top are for?

@FreeMan: The left-hand positions must be able to display respectively N/S for latitude and W/E longitude (like this).

Makes perfect sense now, thanks! I was trying to figure out how they were used to make number display more clear, and never considered letters.

@FreeMan: I realize now that the two groups of digits could be understood as one by IRS, but actually the two groups display a single IRS coordinates (lat, lon). The IRS to display is selected using [SYS DSPL] button. By the way, more than the position can be displayed, the type of information is selected using the [DSPL SEL] button. See slide 52 et seq.

Does the GPS replace the accelerometer or gyroscope during the alignement stage?

Is this alignment a one time event or must this be done regularly?

@trilolil: It must be done each time the gyros are powered, because when restarted the INS doesn't know where is it, nor how it is oriented.


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Can a helicopter be stable without moving the control inputs?
I have only flown helicopters in a flight simulator (X-Plane) and I have found it exceedingly hard to fly in certain regimes (especially hovering and close to hovering) because it seems like there is no point on the controls at which I can stay and not have the helicopter start moving in a direction I want.  As a result, I wind up having to constantly move the control inputs (cyclic and yaw) in small amounts back and forth across where the stability point should be.  On flight simulator forums I have actually seen a couple of people recommend doing this.
But my question is, does this accurately reflect how a real helicopter behaves?  Is it possible to maintain a stable hover or slow speed movement in or out of ground effect once achieved without moving the control inputs much or at all?

I've hovered a Robinson R66 and I can tell you that in a hover it's almost EXACTLY like balancing a broom stick on your hand.Add in collective and anti-torque, and it's more like balancing a broom stick while you also write something with your other hand,while also steering your car with your feet on the steering wheel.It's like learning to play the drums. At the start the broom stick is falling this way and that, and eventually it's second nature and the broom stick just sits as you make tiny corrections.It takes a helicopter student about 8-10 hours of dual instruction to get the hang of it.

Find an X-Plane model of a Kamov helicopter. Those really stay where they are if you let go of the stick. Sikorsky type helicopters are unstable in hover.

Years ago I got a commercial helicopter pilot licence, having learned on a Robinson R22, and a bit on Bell 206 JetRanger. As @John K pointed out, learning to hover seemed to just "click" at around 10 hours. Before that, the tendancy was to overcorrect for everything. Once you "get" it, you tend to just hold the cyclic in one spot and largely ignore the tiny bits of feedback you get, and only react when you really need to. ie there is a real gust of wind from the front, so push the cyclic forward a bit. As for simulators, I've found that they are usually way harder than flying the real thing

The R66 had boosted controls and no force trim, so there is no pressure to work against, not even a force trim bungee spring, at all. It was like I was holding a T shaped piece of wooden dowel upright, and if I let it go it would just fall on the floor.  I was surprised about how sensitive and snappy in roll it was while in forward flight, while being more or less neutrally stable.  Long cross country flights in these things must be very tiring with the level of continuous attention required.

In answer to your main question, Yes that is how they behave.
In theory in a perfectly world, there would probably be some sort of balance point, but in real life there is always something slightly affecting the helicopter causing it to start to drift.
As for how to actually fly, beyond telling you to get real lessons [don't do it], the strategy of constantly moving the controls is something sometimes called "stirring the pot" and in real life is generally a sign of an inexperienced pilot. With training and experience your control inputs will eventually become so small and so far "ahead" of the aircrafts movements that it will look like your not moving your controls. if you are not using full size controls for your sim, this may be impossible to achieve.

Many Hiller helicopters used mechanical stability-augmentation systems which allowed them to be flown completely "hands-off" without loss of control. Those models required positive control inputs on the cyclic to push the helicopter out of its stable zone and make it change course- the opposite of "normal" helicopter pilotage today. I do not know why that control design approach was abandoned.


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Can helicopter rotor produce infrasound?
While watching a Mil Mi-17 helicopter takeoff I experienced strange thing. The helicopter produces a lot of noise even when just starting the engines.
But shortly before taking off the high-pitched sound of the jet engines changed somehow (I guess changing the tilt of the rotor blades) and I felt some sort o pulsing "sound" which I felt rather with my belly than with my ears. Really unpleasant experience.
Could it be infrasound?

Yes. Each rotor blade produces a pressure wave. Here's an example from a Chinook:

At a rotor speed of 4 rotations per second, that's 12 pressure spikes per second, or a frequency of 12 Hz. At frequencies that low, you can hear the individual waves rather than a coherent tone (try this Youtube video on a good set of headphones).
At high levels you tend to feel more than hear this frequency: you can feel the pressure variations on your skin, your eardrums and in your lungs, and you could get resonances in your body.


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if temperature is increased but the volume contained in a area, will an air balloon lift?
I understand the archimedes principle as it relates to water, but am having trouble reconciling a thesis related to a hot air balloon.  If theoretically, I could increase the temperature of air contained in a fixed pocket/container, would the increased pressure exerted upward lift the container?  The archimedes principle would argue against the prospect of lifting the container due to the fact that the container remains the same size and therefore displaces the same amount of air outside.  However, I find it difficult to understand why increased upward pressure on the container would not lift the container.  Can you explain?

This question appears to be off-topic because it is about basic physics and better served on physics.SE.

I am wanting to design an aircraft using elements of buoyancy rather than lift, and am thinking about some basic fundamental questions...so I guess the question is basic, but it is related to a possible aircraft.

@user3736 It is related to aircraft, certainly, but I think what ratchet freak is trying to say is that it is more related to the physics stack, and you are likely to get a better answer there.

The problem with that is that Peter gave a great and understandable answer, leading me to tend to disagree with the hold. The answer given fits the question asked, and is about why a hot air balloon works.

I case of an hot air balloon, the volume of the balloon remains unchanged when the air is heated. But because the heated air expands, part of it escapes from the balloon since it is not closed (the bottom is open). Therefore the weight of the air inside the balloon is reduced. The hot air in the balloon has a lower density that the cool air around it. Therefore the balloon lifts.

Would a closed balloon of unstretchable fabric be used, then the pressure in the balloon would rise but the volume of the balloon would remain unchanged. Therefore the density of the air inside the balloon and the buoyancy would remain the same and the balloon would not lift off.

I get it now thank you...it is not about upward pressure on the balloon it is about the downward pressure causing some amount of air to escape and create lower density in the balloon which is displaced by the higher density outside, forcing the lower density upward.  So, if you could close the balloon and create a vacuum, you could create similar buoyancy of the balloon...actually likely more buoyancy due to the fact of no air vs the less dense air in the conventional balloon.  If this is the case, what are the equations that would govern the amount of weight you could lift?

@user3736: Since pressure does not change, this is the isobaric heating of an ideal gas (air is close enough to be considered ideal). The basic law is called Gay-Lussac's law and postulates that the volume of a gas increases with its temperature. If you lift the temperature from 288°K to 338°K, the volume will increase by 17.36%, or 17.36% of the gas will escape from the envelope.

If instead we close the envelope and instead of heating the gas we remove the air, it seems clear that the envelope will be buoyant and a function of the relationship between amount of displaced air and the weight of the envelope. What are the equations and values of air density governing these relationships?

@user3736 Only if the envelope maintains the same volume when the air is removed. If you remove air from a closed balloon, it will just become a smaller balloon without becoming buoyant.

Assuming the volume remains the same or there is a fixed volume, what volume can support what amount of weight, given that the density inside is zero (a vacuum) and the density outside is our atmosphere at sea level.

@user3736 - Around here we try not to answer questions in comments.  Why not ask that as a new question?

@user3736 While the balloon principles question is at least tangentially aviation-related "What equations govern the buoyancy of a hot-air balloon?" would definitely be more appropriate on the [physics.se] site :)

When you increase the pressure in a closed volume, the increase pressure will act on all parts of the surface. Since both the upward-facing and the downward-facing area is the same, those pressures cancel each other.
If you want to reduce the area on the lower end of the volume, you need to cut a hole in it. Now the upper area is bigger by the area of the hole, and the volume will be forced upwards by a force which is the product of the pressure difference to  outside pressure and the area difference, which is the area of the hole. This kind of lift, however, is not buoyancy, but rocket thrust. Unfortunately, as soon as you open that hole, gas will flow out and the pressure difference will vanish, unless you pump more gas into the volume.

So a hot air balloon behaves similarly, only the air leakage is minimal because the pressure differential between the inner and outer air is minimal, but because the expansive area of the balloon, it works on another principle 'the archimedes principle'...is this the correct way to understand this?

@user3736: A ballon works like a submarine: It displaces air of a mass which is equal to its own mass. You may call it the Archimedes principle, but the normal expression is buoyancy. To compensate for the mass of the envelope and payload, the gas filling the ballon must be less dense than the air outside. This can be achieved by heating the air in the ballon or by using a less dense gas like helium.

This is how I thought it worked, but from what i understand from others is that you do not get less density simply by heating the balloon, rather you must use the increased pressure from heating the gas to allow gas to escape and cause less density inside in order to create buoyancy.

@user3736: Or you allow for the envelope to expand. Yes, this understanding is correct and my description was sloppy, leaving out the escaping part. Since the hot air ballon envelope is open at the bottom, it will always be in a pressure equilibrium with outside, and heating will expand the air, so that some has to escape. The reduction of the air mass inside the envelope by the escaping air is creating the lift which is needed to counteract the mass of the envelope and the payload/basket (whatever hangs under the ballon).